Endoparasitic insects are a critically important element in the biological control of insect vectors of parasitic diseases and other medically important arthropods. In most cases endoparasitism terminates development of the host during its immature stages so that its reproduction is prevented. The long-term goal of this research is to clarify the nature of host-parasite endocrine interactions using the tobacco hornworm and the endoparasitic wasp Apanteles congregatus as a model experimental system. Previous studies have shown that parasitism by A. congregatus causes an abnormal increase in the juvenile hormone (JD) titer of the host larva. The objective of the proposed study is to examine the mechanisms whereby parasitism elevates the host JH titer, causing suppression of host metamorphosis. The specific aims of this project are the following: 1) To investigate effects of parasitism on hemolymph levels of JH-specific esterase and synthesis of this enzyme by the fat body. 2) To assay titers of JH carrier protein in the hemolymph and, if the level is abnormally low, examine fat body capacity to synthesize this protein. 3) To determine if parasitism causes an increase in the rate of JH biosynthesis by the host corpora allata. 4) To clarify whether all terminal stage host larvae show similar endocrine characteristics regardless of their age and size. 5) To experimentally explore possibility that the parasites secrete JH, disrupting the development of the host larva. Results of the proposed experiments will contribute significantly toward increasing our understanding of how development of metazoan parasites affects the endocrine physiology of their host.